Editing Root nodule (section)

==Nodulation==
[[Image:Root-nodule01.jpg|thumb|upright=1.25|right|Cross section through a [[soybean]] root nodule. The bacterium, ''[[Bradyrhizobium|Bradyrhizobium japonicum]]'', colonizes the roots and establishes a nitrogen fixing symbiosis. This high magnification image shows part of a cell with single bacteroids within their [[symbiosome]]s. In this image, endoplasmic reticulum, dictysome and cell wall can be seen.]]
[[File:NitrogenFixingNodulesOnClover.jpg|thumb|upright|Nitrogen-fixing nodules on a clover root.]]
Legumes release [[organic compound]]s as [[secondary metabolite]]s called [[flavonoid]]s from their roots, which attract the rhizobia to them and which also activate [[Bradyrhizobium#Nod genes|''nod'' genes]] in the bacteria to produce [[nod factor]]s and initiate nodule formation.<ref>{{cite journal |last=Eckardt |first=Nancy A. |date=June 2006 |title=The Role of Flavonoids in Root Nodule Development and Auxin Transport in Medicago truncatula |journal=The Plant Cell |volume=18 |issue=7 |pages=1539–1540 |doi=10.1105/tpc.106.044768 |pmc=1488913|bibcode=2006PlanC..18.1539E }}</ref><ref name="Esseling 2003">{{cite journal |last1=Esseling |first1=John J. |last2=Lhuissier |first2=Franck G.P. |last3=Emons |first3=Anne Mie C. |date=August 2003 |title=Nod Factor-Induced Root Hair Curling: Continuous Polar Growth towards the Point of Nod Factor Application |journal=Plant Physiology |language=en |volume=132 |issue=4 |pages=1982–1988 |doi=10.1104/pp.103.021634 |issn=1532-2548 |pmc=181283 |pmid=12913154 }}</ref> These ''nod'' factors initiate '''root hair curling'''. The curling begins with the very tip of the root hair curling around the ''Rhizobium''. Within the root tip, a small tube called the infection thread forms, which provides a pathway for the ''Rhizobium'' to travel into the root epidermal cells as the root hair continues to curl.<ref>{{cite book | first1 = Joan | last1 = Slonczewski | first2 = John Watkins | last2 = Foster | title = Microbiology: An Evolving Science | isbn = 978-0393614039 | edition = Fourth | location = New York | oclc = 951925510 | year = 2017 }}</ref>

Partial curling can even be achieved by ''nod'' factor alone.<ref name="Esseling 2003"/> This was demonstrated by the isolation of ''nod'' factors and their application to parts of the root hair. The root hairs curled in the direction of the application, demonstrating the action of a root hair attempting to curl around a bacterium. Even application on lateral roots caused curling. This demonstrated that it is the ''nod'' factor itself, not the bacterium that causes the stimulation of the curling.<ref name="Esseling 2003"/>

When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: [[cell division]] is triggered in the root to create the nodule, and the [[root hair]] growth is redirected to curl around the bacteria multiple times until it fully encapsulates one or more bacteria. The bacteria encapsulated divide multiple times, forming a [[microcolony]]. From this microcolony, the bacteria enter the developing nodule through the infection thread, which grows through the root hair into the basal part of the [[Epidermis (botany)|epidermis]] cell, and onwards into the [[cortex (botany)|root cortex]]; they are then surrounded by a plant-derived [[symbiosome|symbiosome membrane]] and differentiate into bacteroids that [[nitrogen fixation|fix nitrogen]].<ref>{{cite journal|last1=Mergaert|first1=P.|last2=Uchiumi|first2=T.|last3=Alunni|first3=B.|last4=Evanno|first4=G.|last5=Cheron|first5=A.|last6=Catrice|first6=O.|last7=Mausset|first7=A.-E.|last8=Barloy-Hubler|first8=F.|last9=Galibert|first9=F.|last10=Kondorosi|first10=A.|last11=Kondorosi|first11=E.|display-authors = 6|title= Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis |journal= PNAS |volume=103 |issue=13 |pages= 5230–35 |date= 2006 |doi= 10.1073/pnas.0600912103 |pmid=16547129|id= Online  |bibcode=2006PNAS..103.5230M|pmc=1458823|issn=1091-6490|doi-access=free}}</ref>

Effective nodulation takes place approximately four weeks after [[crop|crop planting]], with the size, and shape of the nodules dependent on the crop. Crops such as soybeans, or peanuts will have larger nodules than forage legumes such as red clover, or alfalfa, since their nitrogen needs are higher. The number of nodules, and their internal color, will indicate the status of nitrogen fixation in the plant.<ref>{{Cite web|url=http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|title=Nitrogen Fixation and Inoculation of Forage Legumes|last=Adjei|first=M. B.|publisher=[[University of Florida]]|access-date=December 1, 2016|archive-url=https://web.archive.org/web/20161202170130/http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|archive-date=December 2, 2016|url-status=dead}}</ref>

Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene). '''Autoregulation of nodulation'''<ref name="Reid">{{cite journal |last1=Reid |first1=DE |last2=Ferguson |first2=BJ |last3=Hayashi |first3=S |last4=Lin |first4=YH |last5=Gresshoff |first5=PM |title=Molecular mechanisms controlling legume autoregulation of nodulation. |journal=Annals of Botany |date=October 2011 |volume=108 |issue=5 |pages=789–95 |doi=10.1093/aob/mcr205 |pmid=21856632|pmc=3177682 }}</ref> controls nodule numbers per plant through a systemic process involving the leaf. Leaf tissue senses the early nodulation events in the root through an unknown chemical signal, then restricts further nodule development in newly developing root tissue. The Leucine rich repeat (LRR) receptor kinases (NARK in soybean (''Glycine max''); HAR1 in ''[[Lotus japonicus]]'', SUNN in ''[[Medicago truncatula]]'') are essential for autoregulation of nodulation (AON). Mutation leading to loss of function in these AON receptor kinases leads to supernodulation or hypernodulation. Often root growth abnormalities accompany the loss of AON receptor kinase activity, suggesting that nodule growth and root development are functionally linked. Investigations into the mechanisms of nodule formation showed that the [[ENOD40]] gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3].